Extragalactic astronomy from the RIT campus

Michael Richmond
Aug 8, 2012

Believe it or not, it's possible to make some contributions
to the field of extragalactic astronomy even from the
light-polluted campus of RIT.
The opportunities arise only rarely, but with a little luck,
and good weather, one can take advantage of them to
help other astronomers with much bigger telescopes
and much better sites.

In August, 2011, just a little less than one year
ago, a supernova was discovered in the nearby galaxy
M101.

Not only was this much closer than the typical
supernova, but it was also discovered very early in
its evolution -- about one day after the explosion began.
This gave astronomers a rare chance to measure properties
of a Type Ia supernova with very high precision.

Let me tell you about our study of this supernova
at RIT. I'll provide some background for those who
aren't members of the supernova community.

A supernova explosion involves the titanic destruction of an
entire star, producing an extremely luminous lightshow for several
weeks or months.
There are several different types of supernovae, distinguished
by features of their spectra.

Massive stars which run out of fuel in their cores and undergo
a catastrophic core collapse can exhibit different types of spectra
and very different light curves.
Type Ia supernovae, on the other hand, are somewhat homogeneous:
they all tend to have similar spectra, similar light curves,
and similar overall luminosity.
That makes them very useful for the study of cosmology,
since we can use them as standard candles*

What exactly causes a Type Ia supernova?
The answer isn't known -- or, more accurately,
there's no agreement on the answer.
One possibility is a single white dwarf
accreting material from a nearby companion until its
mass exceeds the Chandrasekhar limit.
Another model involves the merger of two white dwarfs
which are in a very tight binary system,
again to create a single object with a super-Chandrasekhar
mass.
There is evidence for both models and the discussion continues.

The important thing is that if one can use Type Ia supernova
as standard candles, then they allow us to measure two
very important numbers:

the Hubble constant

We can use the luminosity of Type Ia SNe in different
galaxies to determine their distances.
Spectra of those galaxies reveal the velocities with which
they recede from us.
Plotting recession velocity against distance
yields the rate at which the universe is expanding.

Because Type Ia SNe are so luminous, we can measure their
properties at very large distances.
If they do indeed have the same intrinsic luminosity,
the decrease in their apparent brightness as a function
of redshift reveals the large-scale geometry of the
universe.
Figure taken from
Perlmutter et al., ApJ 517, 565 (1999)

This sounds great -- all type Ia supernovae are exactly the same
brightness! We can use them easily for all sorts of cosmological
tests.

Right? Right?

Well, no, not really.

It turns out that while almost all type Ia supernovae do fall
within a relatively narrow range of limits,
and while a decent fraction of them really DO seem to be nearly
identical,
their properties do span quite a range.
For example, look at these absolute magnitudes:

Some supernovae are more than 1 magnitude (2.5 times) more luminous
than others.
Does that sound like a good standard candle?
No, of course not.

On the other hand, there does appear to be a relationship between
the absolute magnitude of a type Ia supernova and other properties.
On the horizontal axis of the graphs above is the parameter
called "delta m15", which is just the amount by which the supernova
fades in the first 15 days after its maximum light.
It seems that

slowly-declining supernovae are MORE luminous than average

fast-declining supernovae are LESS luminous than average

There are other properties which correlate with luminosity,
and astronomers have devised several methods which attempt
to convert the observed luminosity to a "standard" luminosity.
After making such corrections, it seems that the
truly random scatter among the peak luminosity
of (at least some) type Ia supernovae
is only about +/- 15 percent.

Astronomers who wish to use supernovae for cosmological purposes
need to answer two questions:

What's the best way to correct the observed light curves?

(this will allow one to measure λ)

Once we've corrected the light curves, just what is the
absolute luminosity of a type Ia supernova at maximum light?

This is where the RIT Observatory and I come into the picture,
in our very minor way.
The galaxy M101 is one of the closest big spiral galaxies
to our Milky Way,
being only about 6.7 Mpc (29.1 mag) away from us.

That means that any supernova in M101 will appear much brighter
than the average supernova in our skies.
And, indeed, SN 2011fe was the third-brightest supernova
visible from Earth in the past 25 years (and perhaps in the past 100 years).

The supernova discovery was announced on Aug 24, 2011, at about
7:50 PM Eastern Daylight Time (that's Aug 25, 2011, in Universal Time),
by the
Palomar Transient Factory.

The next night, Aug 25, I started observing the supernova
from the RIT Observatory with our 12-inch telescope
and SBIG ST-8E CCD camera.
Our images weren't very pretty, but they did show the supernova
and several nearby stars of similar brightness which
served as references.
You can see a list of our observing sessions by going to
the RIT Observatory record and scrolling down to the end.

I measured the brightness of the SN in four optical passbands,
using Johnson-Cousins BVRI filters.

At first, the supernova was very blue, compared to the ordinary
reference stars. An image in the B filter is at left,
in the I filter at right. Note that the camera is a LOT more
sensitive to light in the I-band!

I tried to make measurements on every clear night.
The bad news (scientifically) is that the skies in Rochester aren't clear
all that often. The good news (from a mental health standpoint)
is that the skies in Rochester aren't clear all that often.
Over the next 180 days, I was able to acquire data on 50 nights.
The figure below shows all our data, plus that collected
by astronomers at Michigan State University.

As August turned to September, and September turned to
October, M101 gradually set earlier and earlier.
By mid-October, the galaxy was so low at sunset that I could
barely see it.
Fortunately, M101 is nearly circumpolar from our latitude,
so all I had to do was wait six or seven hours for
it to rise again in the east.

In February, 2012, the supernova had faded so much that it was
hard to measure it accurately, especially in the I-band.
Even after co-adding a number of images, the signal was just
too small.
So, I stopped the observations after Feb 20, 2012.

One of the worries in this business is that, as a supernova
fades,
its light becomes contaminated more and more strongly
by other stars or nebulosity in its host galaxy.
That can lead to a systematic error in the photometry
which grows with time.
I checked the HST archive for images of M101 and found
several pictures taken long before the SN exploded.
The closeup below is an image in the I-band equivalent
filter F814W.
The circles are centered on the position of the supernova
with radii 0.5 and 2.5 arcseconds.
The two nearest bright objects are the multiple star "P"
and the single star "Q",
which have apparent magnitudes of I=21.8 and I=22.2, respectively.
Since SN 2011fe had an apparent magnitude of I=15.2 when I
stopped observing it, it was at least 100 times
brighter than these nearby objects,
and so my measurements were not affected significantly.

The story of SN 2011fe is still being told.
Astronomers continue to measure its radiation at
many wavelengths, and many of the observations
have yet to be published.
Let me mention just a few things which seem
clear at this point.

SN 2011fe was a good, useful, "normal" type Ia event

Since this will be the second-best-observed supernova in
history, after SN 1987A,
we are fortunate that it turned out to be one of the
"normal" type Ia events.
That is, it didn't decline very quickly, nor very slowly,
but at a moderate pace:
we measured a drop of 1.21 magnitudes in B-band within
15 days after maximum light.
That puts it in the middle of the pack.

Vinko et al. (2012)
determined the interval from explosion to maximum light
was about 17.2 days, very similar to the value of 17.4 days
for other "normal" type Ia events.

Moreover, there was relatively little interstellar extinction,
in either the Milky Way or M101.
We don't have to make big corrections to the observations
in order to determine the intrinsic properties of the supernova.

SN 2011fe had the expected, "standard-ish" luminosity

M101 is close enough that astronomers can determine its
distance via several methods.
The best estimates yield a distance modulus of about
(m-M) = 29.10 mag, which combined with the observed
apparent magnitude of m(V) = 9.99 and extinction of
A(V) = 0.08 mag, results in an absolute magnitude of
M(V) = -19.18.

Measurements of other supernovae suggest that a type Ia
which declines at the rate of SN 2011fe ought to have
an absolute magnitude of M(V) = -19.19.
Wow --- right on the money.

Our analysis of type Ia SN light curves needs some work

Some type Ia supernova have light curves which decline quickly,
and others decline slowly.
Astronomers have devised a number of methods for measuring
this property and then using it to correct the peak
brightness to a "standard" value.
Two of the most widely used methods are called
"SALT2"
(
Guy et al. 2007 )
and
"MLCS2k2"
(
Jha, Riess and Kirshner 2007 ).
In an ideal world, astronomers would get the same answers
after applying either of these corrections.
Alas, as
Vinko et al. (2012) demonstrate,
we DON'T get the same result.
The process is rather involved, but the bottom line is
that they find

SALT2 yields a distance modulus to M101 of 29.21 +/- 0.07 mag

MLCS2k2 yields a distance modulus to M101 of 29.05 +/- 0.08 mag

These two values don't quite overlap at the one-sigma level.
The gap between them, moreover, is 0.16 magnitudes.
Recall that the intrinsic scatter among type Ia supernova,
after correction, is about 0.15 magnitudes.
If different groups of astronomers use different algorithms
to analyze type Ia SN light curves,
their results may have a significant offset ...

"Visual" and "V" aren't exactly the same

Well, we already knew this.
The spectral response of the human eye is not exactly the
same as that of the astronomical "V" passband,
although they are (not by coincidence) pretty close.
SN 2011fe was bright enough that it was measured by
many visual observers, using their eyes to compare
the supernova to other stars in the field of their eyepiece.
A comparison of the visual and V-band measurements
shows the same relationship seen in other studies:
the human eye is a bit more sensitive (relative to
the V-band) to blue
objects.